I liker this couple more in his "comic-bizarre" side than the serious, or romantic-fluff. But I liket this oneshot a lot.
While Sakura was slightly OOC I felt to Sai IC. And how wrote the lemon was good.
Basically, I found the whole thing soft :)

"Happy Birthday, Sakura!" shouted many voices from all around the particular section of the restaurant that had been reserved for the occasion.

"Thanks everyone!" she shouted back, standing up and taking a swig from the small glass she held in her right hand.

Most of those present at Sakura's birthday party were now in their early twenties. The exceptions were, of course, the senseis and elders she had become close to over the years – Kakashi, Shizune, and even Tsunade herself.

Soon it was the very late evening and it was time for the restaurant to close. The closing announcement by the manager elicited groans from all those who had managed to make it this far.

"Awww!" said Kiba.

"But I'm having so much fun!" said Hinata, off to one side of the group.

"Tell you what guys," started Shikamaru. "How about we just move the party to my place? There's plenty of room and we'll still be far enough from my folks that they won't care about noise."

Shikamaru, as with many shinobi who lived in a clan atmosphere, still lived in the compound owned by his parents. It was quite large and because it enabled him to keep his privacy, he had no intentions of moving any time soon. The Naru compound also had many rooms of all sizes and it could easily accommodate the dozen or so shinobi who still remained at the restaurant celebrating with Sakura.

"Ok!" shouted Sakura.

"Thanks Shikamaru!" echoed Naruto.

"Let's go!" Choji added and was nearly the first person out the door.

Sakura was about to go out the door arm-in-arm with Shikamaru on one side and Choji on the other when she noticed there was still a person hanging back a bit, not quite comfortable with the situation.

"You guys go ahead," she told her two companions. She turned and approached the lone figure.

"Sai, are you ok?" she asked.

"Oh sure, Sakura," he said, not fooling anyone about how uncomfortable he was.

"Are you coming to Shikamaru's?" she asked.

"Uh….I'm not sure," he replied.

"Why not?" she asked.

"Well…," he started. He was obviously holding something back.

"Hmmm?" she prodded.

"Even after all these years, I am still not very comfortable with these types of situations," he admitted. "I'm not even sure how I should act."

"You mean social situations?" added Sakura. "You don't need to worry about anyone here. You should know that they all like you for who you are. We're all your friends."

He let out just the slightest hint of a grin. "I know," he said. "It doesn't make it any easier though. I still feel self-conscious."

"C'mon," she said to him and took him by the arm. "You aren't going to get any more comfortable until you start hanging out with us."

OlOlOlOlOlO

It only took a 10 minute leisurely stroll to get to Shikamaru's house from the restaurant. Sai had greatly enjoyed the walk with Sakura. Not only was she his teammate when it came to being a shinobi, but from a personal perspective, she always seemed to look out for him. When he had first joined Team Kakashi six years ago, they had not gotten along well at all. That was until she found out more about his past in Root, and how he was raised inside the organization and not in a normal home. He was taught not to have emotions. When Sakura finally realized that, she felt bad for him more than anything else. In time, she learned what he was emotionally aware of and what he was not and she never held his social faux pas against him. She took him under her wing and even tried to help him. Other than his older brother in Root who had died so many years ago, Sai had not known anyone else who he cared so much about – and who seemed to take an interest in him as well. She was definitely one of his best friends.

"We're here!" Sakura shouted sliding the door to one side.

"Shhhh!" Shikamaru said putting his index finger up to his lips. "I know we're pretty isolated here in this part of the compound, but let's not press our luck, eh?"

"Sorry," said Sakura, whispering.

"Here," said Shikamaru, handing Sakura two drinks. She, in turn, handed one to Sai.

"Thank you," said Sai.

Shikamaru rejoined Choji on a cushion in the back corner of the room.

Sai looked around. It was the normal group of friends….Choji and Shikamaru….Naruto was lying on Hinata's lap as she sat next to Shino….Kiba, Lee and Ino were talking to one another….Neji and Tenten were talking to each other as well.

Sakura made a move to join in one of the groups when Sai muttered, "Uh, Sakura?"

She turned. "Hmm?"

"Now what?" he whispered, his hand cupping her ear.

"You really are a rookie at this, aren't you?" she asked. "Just have a seat somewhere and join in the conversation. I'm sure there will be a lot of opportunities to listen to stories about missions we've gone on. Sometimes people will play games. Just….just stay away from any couples that break off from everyone else."

Sai must have given her a confused look after that last statement.

Sakura sighed. She realized that she would need to explain so he would fully understand.

"When two people are boyfriend and girlfriend, often in these types of situations, they will…uh….go off on their own and enjoy each other's company," she explained. "In other words, they want to be alone so they can do things that only boyfriends and girlfriends do."

Sai wasn't stupid – just naïve. He knew about men, women and sex. He just never knew this was the type of situation that led to it. For all he knew, they just said "meet me in the bedroom at 10 o'clock" and it just kind of happened. Though he was well aware that it had never happened to him.

I wonder what its like to be in that kind of relationship?

"Uh, ok. Thanks," he replied to the pink-haired beauty. He decided the best place to hang out at the current time was over by Shino. He was sitting with Naruto and Hinata after all. And he was also naturally a quiet person. Sai thought then he wouldn't be so out-of-place.

Sakura wandered over to Choji and Shikamaru and took a seat and started talking.

To be quite honest, Sai wasn't enjoying himself. Even after only 15 or so minutes where he was, he found that no one was really talking except for Naruto and Hinata, and much of their conversation was just them flirting with one another. Shino barely said a word. I knew he wasn't going to talk much, but I guess that was a mistake on my part for thinking that I'd be the most comfortable here.

Sai looked up and saw Sakura laughing while Choji took a swig of a drink and Shikamaru smiled at something Sakura had just said.

He likes her, Sai thought to himself. At least, I think that he does. He had definitely gotten better at reading people, but was still not confident in any of his abilities related to interpersonal relationships. Enough of this.

Sai got up and wandered over to where Sakura sat and took a seat by her.

"Remember that time that Asuma-sensei caught us in the boys' bathroom?" asked Choji.

"Yeah, I thought he was going to kill us for sure," noted Shikamaru.

"That's nothing," added Sakura. "Once, Sasuke did the very same thing to Kakashi-sensei. You should have seen the look on sensei's face when he caught him."

"Ooohhh!" laughed the boys at the same time.

Sai wasn't following the conversation very well, but still smiled to try to fit in a bit more with the group he was sitting with.

"What about you, Sai?" asked Sakura. "Have you ever played any practical jokes on any of the senseis?"

Sai tried to think of a good comeback, but after a long, awkward pause, he had nothing. "Uh, no," he replied. "Sakura, you honestly believe that a guy like me could do that?" He tried to laugh it off a bit.

Sai let out a forced laugh. Thankfully, the conversation was soon diverted away from him again. He looked around the room for a brief moment and noticed that Neji and Tenten were now kissing and Ino was giving Lee dirty looks.

Kiba got up and walked over to where Sai and Sakura were sitting.

"Can you guys do me a favor?" Kiba asked lowly.

All eyes soon turned to acknowledge him.

"Can you….get Lee over here? The guy seriously cannot take a hint."

Sakura smiled. "Lee!" she shouted across the room. "I have something to ask you!"

"Thanks Sakura, you're a lifesaver," remarked Kiba. He turned to return to Ino once he saw Lee stand up.

"You owe me one," Sakura said to Kiba. Then she winked.

"Yes, Sakura?" Lee said as he approached.

"Oh Lee, I – uh – was just wondering if you had found yourself a girlfriend yet?" Sakura asked him.

That was the wrong question to ask him, however. Rock Lee still had a pure-hearted crush on the pink-haired shinobi and even after all these years, he still thought he might have a chance with her. But by her asking the question, she was reinforcing the fact that she wasn't interested. His expression was one of sadness.

"Oh Lee, don't give me that look," said Sakura. She took his head and laid it on her shoulder lovingly.

He looked up at the other three guys and gave them the thumbs up.

Sakura saw and rolled her eyes.

They all knew it wasn't anything personal. They liked to play, flirt and tease like this. This was one of the things that made them all such good friends. Sai was included in all of that, though he knew that he was probably the least comfortable or knowledgeable about all this stuff. He was going to keep trying to figure it all out though.

The friends continued to talk, drink, and laugh into the mid-morning hours. The couples were among the first to leave.

When Neji and Tenten wished everyone a good night and left, Sakura snickered, "I wonder what they're going to do now."

Everyone else also snickered…except Sai. He was still a confused, but knew it would be embarrassing to ask, so he kept his mouth shut.

It wasn't too much later that Kiba and Ino and Naruto and Hinata also said goodnight to the group.

It was close to 4am when Sakura finally yawned. "I can't keep my eyes open," she said while stretching. "

It started out weird - no real description, and a strange sense that there was little to no direction. It wasn't until the beginnings of the lemon itself that there was some obvious path here. But I like that you didn't rush the scenes into the lemon though, giving some dialogue to their interactions, and I think you get into Sai's head well enough; he's definitely at an emotional point I'd imagine he would be post-Naruto Shippuden. I love that he and Sakura were already so comfortable with each other too. Sakura was a little OOC, but it came out okay against Sai's personality. A sexually curious Sai is a nice read. :)

I thought Sai was very in character, Sakura maybe less. However, I could not tell since I don't understand the girl very well. The dialogue between the two of them was entertaining. The lemon part was nicely done. I feel like as soothing it was overall, it felt very mechanic and not so very intimate. I guess we're supposed to feel that way because Sai is the very definition of social awkwardness. Nevertheless, I did not feel that way because of Sai but because of Sakura. The high point of this fiction is that you know how Sai's mind "works".
It is well-written in my opinion. I like that there are not too many descriptive parts, because it works so well for this fic. There are a couple of typos, like this one towards the end:" He thoughts were interrupted, however, by a change in the way Sakura felt to him." It has to be because you did not see it because the writing is flawless.
Lastly, thanks for writing this SaixSaku's oneshot. Too bad this pairing falls so far behind SasuxSaku's popularity. There is true potential there.